Agriculture Reference
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unicellular pointed trichomes were less susceptible to insect damage than
genotypes possessing bicellular blunt trichomes [94]. Satish et al. [94]
identifi ed eight QTLs (quantitative trait loci) for trichome density using
sorghum, two of which were specifi c for upper leaf surface, the remaining
six specifi c to the lower leaf surface. Four QTL were identifi ed in maize
for trichome density by Lauter et al. [98], several of which were syntenic
to those determined in sorghum. By breeding for increased trichome den-
sities and benefi cial morphology, improved resistance to insect herbivory,
specifi c per host-herbivore relationship can be achieved.
Although several epidermal traits have been correlated to improved re-
sistance to insect herbivory, recently much of the research has focused on
molecular plant responses that improve tolerance to insect feeding. Sev-
eral phytohormones are involved in a plant's defense response to insect
herbivory, either directly or indirectly, however jasmonates (JA) appear
to have the strongest involvement in response to insect feeding. Using
hormone-related mutants of
Arabidopsis
, Abe et al. [99] found that JA
played the most signifi cant role in defense and tolerance to thrip feeding.
Although JA plays an important role in anti-herbivory defense, it does not
act alone. Brassinosteroids (BR) have been shown to have a signifi cant,
yet negative interaction with JA in the stimulation of herbivory defenses
in tomato, specifi cally trichome development and regulation of proteinase
inhibitors [100]. More recently, progress has been made to better under-
stand BR and JA crosstalk involved with herbivore defense (Figure 3).
Meldau et al. [101] determined that the SGT1 protein is involved in the
accumulation of JA and when absent herbivory defense is reduced. In ad-
dition, BAK1, a co-receptor involved in BR signaling, also plays a role in
JA accumulation, as well as involvement in altering levels of proteinase
inhibitors [102]. By breeding plants with increased sensitivity to JA or
insensitivity to BR, genotypes can be developed that will help improve in-
sect resistance, reducing the need for heavy insecticide applications, while
increasing the yield and sustainability of low-input systems.
The development of varieties resistant to common, crop specifi c patho-
gens of economic importance is essential to reducing the pesticides needed
and the energy consumed by low-input systems to improve yield. Bacteria
and fungi, like insects, are diffi cult to breed for based on the variety of
way in which they infect crops and reproduce, as well as their ability to
